1. Field of the Invention
The present invention relates to a cross-head die and a method for manufacturing a resin structure reinforced with long fibers using the cross-head die. More particularly, the invention relates to a cross-head die which is used for impregnating a continuous fiber bundle with a thermoplastic resin melt by passing the fiber bundle through a vertically meandering passage in the die (impregnation die), in which a plurality of top portions and a plurality of bottom portions of the meandering passage are designed to have a variety of heights. It also relates to a method for manufacturing a resin structure reinforced with long fibers using the cross-head die.
2. Background of the Invention
Resin structures reinforced with long fibers made of parallel aligned fiber bundles and a thermoplastic resin permeating the fiber bundles are primarily processed into elongated pellets, which are used for manufacturing various molded products which require mechanical strength. The resin structures are prepared by passing a fiber bundle which is typically of continuous glass fibers through a passage in a cross-head die, impregnating the fiber bundle with a thermoplastic resin in the die, and shaping the impregnated fiber bundle to a desired shape such as that of a pellet using a shaping die. Such a technique is disclosed, for example, in U.S. Pat. No. 4,39,387, Japanese Patent Application Laid-open (kokai) No. 3-272830, and Japanese Patent Application Laid-open (kokai) No. 63-264326.
In the process of manufacturing resin structures reinforced with long fibers disclosed in the above references, a plurality of vertically meandering passages provided in the cross-head die have a regular pitch and height. Therefore, a tensile force applied to the continuous fiber bundle at the top or bottom portions of the passage when the fiber bundle contacts such portions is always identical. In this case, impregnation performance is improved as the number of top and bottom portions passed by the fiber bundle increases.
Thus, although a conventional cross-head die provides improved impregnation performance in proportion to the number of top and bottom portions, it has a disadvantage that excessive tension is applied to the fiber bundle, and this drawback is considerable particularly at the top or bottom portions close to the outlet of the cross-head die. Excessive tension causes fuzzy fibers in a fiber bundle, and in addition, sometimes causes strand breakage (broken fiber bundle) when the fiber bundle passes through the die, or causes entangled fibers on products such as pellets. Needless to say, when such defective products are molded, the resulting molded products will have poor quality.
The present inventors carried out extensive studies in an attempt to solve the above problems involved in the manufacture of resin structures reinforced with long fibers in which a continuous fiber bundle is impregnated with a thermoplastic resin melt while passing the fiber bundle through a cross-head die. As a result, they found that it is very effective to vary the height of the vertically meandering top or bottom portions of the passage in a cross-head die for avoiding inconvenient breakage of fiber bundles and for enhancing the impregnation performance. The present invention was accomplished based on this finding.